This invention relates to normally tacky and pressure-sensitive adhesive sheet material and is particularly concerned with repositionable products.
In the mid-1970's applicant's assignee introduced repositionable adhesive tapes and note papers, which found an immediate acceptance and today provide a substantial volume of business. The adhesive utilized in these products, typically present in a narrow band adjacent one edge of the sheet material, consists essentially of an adhesive binder layer containing numerous infusible, solvent-dispersible tacky elastomeric microspheres that typically have a diameter of 50-75 micrometers. In another embodiment of this type of product, bulletin boards are surfaced with sheet material bearing the same repositionable adhesive, so that notices, pictures, clippings, etc. can be temporarily adhered, removed, and repositioned; see, e.g., U.S. Pat. No. 3,857,731.
Products of the type described in the preceding paragraph can be adhered to almost any substrate, including paper (even such weak paper as newsprint), readily removed without delaminating the substrate, and subsequently repositioned. The force required to peel these products from a paper substrate is maintained in the approximate range of 8 to 80 (preferably 10 to 50) grams per centimeter width; products having significantly lower peel adhesion (e.g., less than 8 g/cm width) generally lack the ability to resist inadvertent removal, often falling off. Products having significantly higher peel adhesion (e.g., more than 80 g/cm) on the other hand, tend to tear or delaminate weak papers at normal removal rates. Application of the adhesive coating so as to achieve consistent peel adhesion values requires considerable knowhow, and the adhesive compositions themselves are relatively expensive to manufacture.
Numerous attempts have been made to achieve the results obtainable with microsphere adhesives by substituting more conventional pressure-sensitive adhesives (psas), which typically have continuous-coat (100% coverage) peel adhesion values, when applied to untreated paper, on the order of 100-1000 g/cm. width. (Representative adhesion values include the following: matte finish acetate tape, 107 g/cm; masking tape, 227 g/cm; book repair tape, 443 g/cm; package sealing tape, 830 g/cm; filament tape, 937 g/cm.) One approach has been to use a psa having inherently low tack, but it has been found difficult, if not impossible, to maintain consistent peel adhesion values, the nature of the note paper backing and the thickness of the adhesive layer both profoundly affecting performance. Another approach has been to apply a thinner coating than normal, it being recognized that this will reduce peel adhesion. Unfortunately, however, it is almost impossible to prepare consistent coatings having removal values in the 10-50 g/cm range; even when this can be done, adhesion will frequently increase upon extended contact to a degree sufficient to cause tearing or delamination upon attempted removal.
Others have tried to obtain the desired degree of adhesion by locally deactivating portions of a full coating of a psa, as taught in U.S. Pat. No. 2,515,423, but this technique has likewise proved difficult to control. The same problems are encountered in attempting to locally mask portions of a full coating of a psa, as taught in U.S. Pat. Nos. 1,944,834, 2,000,475, 3,900,642, 3,967,624, and 4,063,559, as well as in British Patent No. 1,541,311.
The prior art has also suggested applying pressure-sensitive adhesive to only portions of a backing, leaving other portions free from adhesive. Thus, it has been proposed to make spaced adhesive stripes extending in either the machine direction (U.S. Pat. No. 2,349,709) or the cross direction (U.S. Pat. Nos. 2,386,731 and 3,811,438), as well as products having spaced adhesive spots (U.S. Pat. Nos. 3,174,888 and 3,741,786). By and large, however, the spots were so large that attempted removal of the tape from newsprint will cause tearing or delamination. Prior to the present invention, then, it is believed that there has never existed a product having performance equivalent to that of the microsphere adhesives referred to above, but made with conventional pressure-sensitive adhesives.